Boxer's beginning with crude left in fight (5)
I believe the answer is:
brawl
'in fight' is the definition.
(I've seen this before)
'boxer's beginning with crude left' is the wordplay.
'beginning' says to take the initial letters.
'with' means one lot of letters go next to another.
'crude' becomes 'raw' (synonyms).
'left' becomes 'l' (common abbreviation).
The first letter of 'boxers' is 'b'.
'b'+'raw'+'l'='BRAWL'
(Other definitions for brawl that I've seen before include "Scuffle, unseemly fight" , "Fracas" , "Affray" , "Noisy quarrel" , "Riotous fight" .)